In recent years, in order to meet commercial needs for accelerative shifts to color image formation in office use, for more high-definition images adaptable to the market of graphics, for higher speed adaptable to light-duty printing and for something else, two-component developers used in electrophotography are sought to achieve much higher image quality and higher stability from an aspect of performance.
In the present state of affairs, electrophotographic carriers making up such two-component developers are chiefly held by coated carriers obtained by coating ferrite particle surfaces or magnetic material dispersed resin core surfaces with a coating resin. Coat layers play roles of, e.g., making toners have stable charge quantity distribution and keeping electric charges from being injected from the electrophotographic carrier into a photosensitive member. However, studies have still not sufficiently been made on the coating of electrophotographic carrier core surfaces with the coating resin, and there still remain many problems or subjects concerning how to effect the coating uniformly.
Conventional methods for producing electrophotographic carriers include what is called a dipping method in which electrophotographic carrier cores and a coating resin solution are stirred and the latter's solvent is evaporated with stirring to coat the electrophotographic carrier core surfaces with the coating resin. A method is also available in which a coating resin solution is sprayed by means of a spray nozzle on electrophotographic carrier cores while forming them into fluidized beds, to coat the electrophotographic carrier core surfaces with the coating resin. Such wet-process coating methods have been prevalent.
The wet-process coating methods, however, have had a problem that the electrophotographic carrier particles tend to come to coalesce when the solvent evaporates. If an electrophotographic carrier the particles of which have once come to coalesce is disintegrated as a result of stirring, the electrophotographic carrier core surfaces may come bare to faces of such disintegrated particles, so that what is called a leak tends to occur which is a phenomenon that electric charges come injected from the electrophotographic carrier into the photosensitive member as mentioned above. If such a leak occurs, the surface potential of the photosensitive member may converge on development bias to make any development contrast not securable to cause blank areas in images. In addition, the fact that the electrophotographic carrier core surfaces come bare makes it unable for a toner to retain electric charges especially in a high-temperature and high-humidity environment, so that faulty images and the like tend to come about because of a low chargeability of the toner after its leaving over a long period of time.
In addition, in the wet-process coating methods, a low yield tends to result if the electrophotographic carrier particles come to coalesce. Usually, classification is carried out at the final stage of electrophotographic carrier production steps. This is because electrophotographic carrier particles having coalesced and not disintegrated come to be removed. Further, a drying step is necessary which is to remove the solvent completely, and this can be a factor of the elongation of tact time. Thus, there still remain many problems on the wet-process coating methods from an aspect of production as well.
Accordingly, a dry-process coating method is proposed as a method which can resolve the problems the above wet-process coating methods have. For example, a method is disclosed in which a powdery coating treatment material is mixed and agitated by means of a high-speed agitating mixer, during which the coating treatment is thermally carried out at glass transition point (Tg) or more of a coating resin contained in the coating treatment material, to obtain a carrier (Japanese Patent Laid-open Application No. H09-160307). However, in this method, the whole interior of an apparatus is heated with a jacket so that the whole coating treatment material can have a temperature not lower than the Tg of the coating resin contained in the coating treatment material, and hence the electrophotographic carrier particles tend to come to coalesce as stated above. Thus, this method is still unsatisfactory in that the particles should uniformly be coated.
A method is also proposed in which the dry-process coating is carried out by mechanical impact force (Japanese Patent Laid-open Application No. S63-235959). For example, a method is disclosed in which a surface treating apparatus having a rotor and a liner is used to coat the surfaces of magnetic material particles with resin particles having a particle diameter of 1/10 or less the magnetic material particles. In this method, the resin particles are dispersed on carrier particle surfaces by using an apparatus different from the apparatus for coating treatment, thus the method is disadvantageous in that it additionally requires the apparatus for dispersion. Where the apparatus for dispersion is not used, the resin particles are kept to stand liberated from carrier cores, thus it is difficult to well carry out the treatment to coat carrier core surfaces with the resin particles. In addition, even though the resin particles are made to adhere to carrier core surfaces by using an apparatus different from the apparatus for coating treatment, any excess resin particles may be left to stand liberated when the resin particles are fed in such a large quantity that they can not completely adhere to the carrier core surfaces, and hence it is difficult to carry out the coating treatment uniformly. Thus, this method gives a restriction on the coating quantity when fed, and may make it difficult to control the charge quantity of the toner or keep electric charges from being injected from the electrophotographic carrier into the photosensitive member.
As a powder treating method making use of mechanical impact force, a powder treating method is also proposed in which a strong impact force conventionally not achievable is applied making the most of an advantage a rotary blade type apparatus has (Japanese Patent Laid-open Application No. 2005-270955). According to this method, treatment can variously be carried out not only for mixing and drying particles but also for making particles composite (fusing), particle surface modification, particle surface smoothing, particle shape control (making particles spherical) and so forth. However, in order to make this method usable to carry out the treatment to coat electrophotographic carrier core surfaces with a resin composition by dry-process coating, studies have still not sufficiently been made on treatment conditions and so forth.